1. Field of the Invention
This invention relates to an active matrix circuit for liquid crystal displays (liquid crystal display is hereinafter referred to as LCD) for driving liquid crystal pixels arranged in a matrix on a liquid crystal display pannel.
2. Description of the Prior Art
The active matrix drive method has been known as one of the desirable methods for driving LCDs. The active matrix drive type LCD comprises a plurality of scanning lines extending in an X direction, a plurality of signal lines extending in a Y direction, and a plurality pairs of a pixel and a switching element disposed at crosspoints of the scanning and signal lines. Each pixel is driven by each respective switching element. The switching element may be a thin film transistor (TFT, hereafter) or a non-linear device such as a varistor, a metal-insulator-metal (MIM), a diode ring, a back-to-back diode and a MOS transistor diode.
A TFT array as shown in FIG. 12 is known as a typical active matrix circuit for LCDs, as disclosed, for example, in Japanese Laid-open Patent Application No. 60-192369 Referring to FIG. 12, C.sub.11, C.sub.12, . . . and C.sub.21, C.sub.22, . . . are liquid crystal pixels, T.sub.11, T.sub.12, . . . and T.sub.21, T.sub.22, . . . are TFTs, X.sub.1, X.sub.2, X.sub.3, . . . are scanning lines, and Y.sub.1, Y.sub.2, Y.sub.3, . . . are signal lines. Each TFT has its gated connected to a respective scanning line, and has its source connected to a respective signal line, and has its drain connected to one terminal of a respective pixel. The other terminal of the respective pixel is connected to ground. Each pixel acts as a capacitor. An auxiliary capacitor may be connected in parallel with each pixel.
Selection pulses P.sub.1, P.sub.2, P.sub.3, . . . as shown in FIG. 13 are respectively supplied to the scanning lines X.sub.1, X.sub.2, X.sub.3, . . . . In FIG. 13, T and M respectively represent a frame period and the number of scanning lines. When a scanning line X.sub.1 is selected by a selection pulse P.sub.1, the TFTs T.sub.11, T.sub.12, . . . connected to the scanning line X.sub.1 become conductive between the respective sources and drains, (or turn on), so that signal voltages on the signal lines Y.sub.1, Y.sub.2, . . . are respectively supplied to the pixels C.sub.11, C.sub.12, . . . which are respectively connected to the TFTs T.sub.11, T.sub.12, . . . . When the selection pulse P.sub.1 disappears, the TFTs T.sub.11, T.sub.12, . . . connected to the scanning line X.sub.1 become non-conductive (or turn off) and the signal voltages supplied to the pixels C.sub.11, C.sub.12, . . . are held until the scanning line X.sub.1 is again selected. In this way, the signal voltages on the signal lines can be accurately transferred to the pixels via the respective TFT switching elements, so that a high contrast display can be realized.
However, in the above described configuration, when the number of scanning and signal lines are increased it would be very difficult to produce all of the TFTs without a defect. As shown in FIG. 14 which shows a sectional view of a TFT, the source 2 and drain 3 of the TFT are isolated from the gate 1 via an insulating layer 8 and a semiconductor film 9. In such a structure, the gate and the drain or the gate and the source would be short-circuited due to a pin hole or other defects caused during manufacturing process of the TFT array. Particularly, if the gate and the source are short-circuited, all of the TFTs which are connected to the scanning and signal lines connected with the defective TFT will not properly operate. This is a serious defect called a "line defect". Furthermore, the source and the drain would also be short-circuted due to a failure of photo-lithography. In this case, the liquid crystal cell connected to the defective TFT will operate abnormally, which is called a "point defect".
A conventional LCD panel using non-linear switching elements is disclosed, for example, in "Varistor-Controlled Liquid Crystal Displays" by D. E. Castleberry, IEEE Transactions on Electron Devices, Vol. ED-26, No. 8, pp. 1123-1128 (August 1979). This LCD pannel comprises a first substrate mounting thereon a plurality of row electrodes (scanning lines), a plurality of first pixel electrodes and a plurality of non-linear switching elements (varistors in this case), and a second substrate mounting thereon a plurality of column electrodes (signal lines) and a plurality of second pixel electrodes, the first and second substrates being confronted with each other so that a plurality of liquid crystal cells are respectively disposed between the first and second pixel electrodes.
FIG. 17 shows an equivalent circuit of the above noted LCD panel, in which S.sub.11, S.sub.12, . . . and S.sub.21, S.sub.22, . . . are the non-linear switching elements, 7 is the liquid crystal cell, and 10 and 11 are respectively the first and second pixel electrodes. Each non-linear switching element has one terminal connected to a respective scanning line, and has another terminal connected to the first pixel electrode of a respective pixel. The second pixel electrode of each pixel is connected to a respective signal line. The non-linear switching element has a voltage-current characteristic as shown in FIG. 18 in which V.sub.b is a threshold voltage.
Selection pulses P.sub.1, P.sub.2, . . . which are negative voltage pulses as shown in FIG. 19 are respectively supplied to the scanning lines X.sub.1, X.sub.2, . . . . In FIG. 19, T and M represent a frame period and the number of scanning lines, respectively. A signal voltage supplied to each of the signal lines Y.sub.1, Y.sub.2, . . . is set to be smaller than the threshold voltage V.sub.b of the non-linear switching element so that the signal voltage is not supplied to non-selected pixels. When a selection pulse P.sub.1 is supplied to the scanning line X.sub.1, i.e., the pixels C.sub.11, C.sub.12, . . . connected to the scanning line X.sub.1 through the respective non-linear switching elements S.sub.11, S.sub.12, . . . are selected, the voltage difference between the signal line X.sub.1 and each signal line to which a signal voltage is supplied becomes the sum of the peak voltage of the selection pulse P.sub.1 and the signal voltage, so that a voltage which is the difference voltage between the sum voltage and the threshold voltage is supplied to the respective pixel. After the selection pulse P.sub.1 disappears, the voltage across each of the pixels C.sub.11, C.sub.12, . . . is held until the scanning line X.sub.1 is again selected (in the next frame).
As described above, in the LCD pannel using non-linear switching elements, each signal voltage is accurately and independently transferred to a pixel, so that a high contrast display without crosstalk can be realized. The other devices usable as the non-linear switching element are a diode ring, a back-to-back diode, a diode-connected MOS transistor, which are respectively shown in FIGS. 16(a), (b) and (c), and a MIM.
However, in the above described configuration of the LCD panel, if the number of scanning lines (i.e. the number of pixels) is increased, it would be very difficult to produce all of the non-linear switching elements without defects. Some non-linear switching elements would be short-circuited or open-circuited. If one of the non-linear switching elements is defective the pixel connected to the defective switching element is not supplied with a normal signal voltage so as to cause the so-called "pixel defect".